The present invention relates generally to cooling apparatuses, and more particularly to a cooling apparatus for cooling an optical element used in an exposure apparatus that exposes an object, such as a single crystal substrate for a semiconductor wafer (plate or ball), and a glass plate (wafer) for a liquid crystal display (LCD). The present invention is particularly suitable, for example, for an exposure apparatus that uses ultraviolet light and extreme ultraviolet (“EUV”) light as a light source for exposure.
A reduction projection exposure apparatus has been conventionally employed which uses a projection optical system to project a circuit pattern formed on a mask (reticle) onto a wafer, etc. to transfer the circuit pattern, in manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in photolithography technology.
The minimum critical dimension to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Along with recent demands for finer semiconductor devices, a shorter wavelength of ultraviolet light has been promoted from an ultra-high pressure mercury lamp (i-line with a wavelength of approximately 365 nm) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm).
However, the lithography using the ultraviolet light has the limit to satisfy the rapidly promoting fine processing of a semiconductor device, and a reduction projection optical system using extreme ultraviolet (“EUV”) light with a wavelength of 10 to 15 nm shorter than that of the ultraviolet (referred to as an “EUV exposure apparatus” hereinafter) has been developed to efficiently transfer a very fine circuit pattern of 0.1 μm or less.
The light absorption in a material remarkably increases as the wavelength of the exposure light becomes shorter, and it is difficult to use a refraction element or lens for visible light and ultraviolet light. In addition, no glass material exists in a wavelength range of the EUV light, and a reflection-type or cataoptric optical system uses only a reflective element or mirror.
The mirror does not reflect all the exposure light, but absorbs the exposure light of 30% or greater. The absorbed exposure light causes residual heat, deforms a surface shape of the mirror, and deteriorates its optical performance, in particular, imaging performance. Therefore, the mirror is made of a low thermal expansion glass, for example, having a coefficient of linear expansion of 10 ppb, so as to reduce a mirror's shape change as the temperature changes.
The EUV exposure apparatus is used for exposure of a circuit pattern of 0.1 μm, and has strictly limited critical dimension accuracy. Therefore, the mirror's surface shape is permitted to have deformation of only about 0.1 nm or less. Therefore, even the mirror's coefficient of linear expansion of 10 ppb would cause the temperature to rise gradually and the mirror's surface shape to change. For example, when the mirror has a thickness of 50 mm, the mirror's surface shape changes by 0.1 nm as the temperature rises by 0.2° C.
A common method cools a mirror 5000, as shown in FIG. 9, by coupling a joint 5100 to the mirror 5000, connecting a water pipe 5200 to the joint 5100, and supplying coolant, such as the water, to a channel 5300 formed in the mirror 5000. Here, FIG. 9 is a view of a conventional method for cooling the mirror 5000, wherein FIG. 9A is a schematic transparent plane view of the mirror 5000, whereas FIG. 9B is a schematic sectional view of the mirror 5000.
The low thermal expansion glass for the mirror 5000 is, however, so soft that a force generated in coupling the mirror 5000 to the joint 5100 deforms the surface shape of the mirror 5000 by 0.1 nm. In addition, pressure to flow the coolant through the channel 5300 in cooling the mirror 5000 also deforms the surface shape of the mirror 5000.
It is conceivable to use convective heat transfer to cool the mirror in a non-contact manner without applying a force to the mirror, such as blowing gas to the mirror. However, the EUV exposure apparatus cannot use air because it maintains an atmosphere of an exposure optical path to be high vacuum, for example, about 1×106 [Pa] so that a reaction between the residual gas component in the exposure optical path, such as polymer organic gas, and EUV light may not contaminate a mirror surface and lower its reflectance.